1. Field of the Invention
The present invention relates to musical tone synthesis techniques. More particularly, the present invention relates to what is known as "physical-modeling synthesis" in which tones are synthesized in accordance with the mechanisms which occur in natural musical instruments. Music synthesis based on a physical model is gaining on currently dominant methods such as "sampling" (or "wave table") synthesis and frequency modulation (FM) synthesis. Such synthesis techniques are particularly useful for simulation of wind instruments and string instruments. By accurately simulating the physical phenomena of sound production in a natural musical instrument, an electronic musical instrument is capable of providing high quality tones.
2. Description of Related Art
In the case of a string instrument, the structure for synthesizing tones typically includes a filtered delay loop, i.e., a closed loop which includes a delay having a length corresponding to one period of the tone to be generated and a filter contained in a closed loop. An excitation signal is introduced into the closed loop and circulates in the loop. A signal may be extracted from the loop as a tone signal. The signal will decay in accordance with the filter characteristics. The filter models losses in the string and possibly at the string termination (e.g., nut and bridge in a guitar).
In an actual stringed instrument, the string is coupled to a resonant body and the vibration of the string excites the resonant body. In order to accurately model a natural musical instrument, therefore, it has been necessary to provide a filter at the output of the filtered delay loop. To obtain high quality sound, it has been necessary to follow the string output by a large and expensive digital filter which simulates the musical instrument body. The excitation signal generally takes the form of white noise or filtered white noise. Alternatively, a physically accurate "pluck" waveform may be provided as an excitation to the closed loop, which results in more accurate plucked string simulation.
A tone synthesis system as described above is illustrated in FIG. 1. A filtered delay loop is formed of a delay element 10 and a low pass filter 12. An excitation source (e.g., a table) 14 provides an excitation signal into the loop via an adder 16. The contents of the excitation table may be automatically read out of a memory table in response to a trigger signal generated in response to, e.g., depression of a key. The excitation signal which is inserted into the filtered delay loop circulates and changes over time due to the filter operation. A signal is extracted from the delay loop and provided to a body filter 18. For high quality instrument synthesis, a complicated and expensive body filter (typically a digital filter) or additional filtered delay loop is required.
The tone synthesis system illustrated in FIG. 1 may be implemented in hardware, although it is somewhat more common to implement the tone generation technique in software utilizing one or more digital signal processing (DSP) chips. The system of FIG. 1 is capable of very high quality tone synthesis. However, it has the drawback of requiring a complex and expensive digital filter which simulates the instrument body.